LED lighting is becoming more and more popular as manufacturing costs are reduced to offer affordable LED lamps for many fixture types. In one widespread type of LED lighting device for indoor or outdoor lighting, a rectifier is used to rectify a mains input, and an LED driver delivers the desired voltage and current levels to an LED arrangement comprising one or more strings of LEDs. For cost reasons, many LED drivers use a single-stage power conversion architecture, even if the driver requires a high power factor. However, this can lead to a high LED ripple current. The ripple current might be compensated to some extent by a storage capacitor connected in parallel to the LEDs, but cannot be corrected entirely due to the limited dynamic resistance of the LEDs. Due to the increasingly efficient LED technology, that dynamic resistance also tends to decrease further in time. The pronounced ripple current can also adversely affect the luminous efficiency of the LEDs.
In a known approach to correcting this problem, a linear post-regulator is connected at the output of the LED arrangement and serves to control the LED current while the voltage supply to the LED arrangement is controlled by the power converter. Existing linear post-regulators can be relatively inexpensive, but are generally associated with an increase in losses. A linear post-regulator generally comprises a transistor that acts as a controllable resistance controlling the current through the LEDs. Some power is therefore dissipated by this controllable resistance. To cope with the resulting heat, additional or larger heat sinks are required, adding to the overall cost of a lighting application. Another disadvantage of the known types of linear post-regulator is that the level of flicker on the light output can increase, since the storage capacitor can degrade down to 50% of its initial value during the lifetime of such a lighting arrangement. The flicker in such a system depends to a large extent on the LEDs' dynamic resistance. Particularly, standalone LED drivers will have to cope with a range of LED types and thus a range of dynamic resistances.
It is difficult and/or costly to reduce post-regulator losses. Some ways of addressing the problem are to improve thermal management by using a larger heatsink, as already mentioned, however this may unfavourably increase the overall size of the product and adds to the overall costs. Alternatively, a switched-mode converter may be used as the second power stage, which can achieve an almost negligible flicker but which is significantly more expensive and bulky and also adds losses. In another approach, a larger storage capacitor is used to avoid the need for a post-regulator; however the physical size of a capacitor increases with its value, and the cost increases also. When no corrective measures are taken, the lamp may eventually exhibit noticeable flicker.
Therefore, it is an object of the invention to provide an improved linear post-regulation that avoids the problems outlined above.